Apologies if it's in bad taste to reply to my own post, especially because it's about the product I'm working on, but here are some of Dropbox's differences/improvements over what people typically hack together themselves:

- syncs continuously/watches the FS for file changes (no cron jobs needed -- things usually sync as quickly as they can be sent)
- does binary diffing and only sends deltas (compressed & over SSL)
- transparently archives past versions of all files (i.e. undelete/infinite undo)
- syncs across any number of machines
- lets you get to your files from the web
- some more info @ http://venturebeat.com/2007/08/16/the-y-combinator -list/

We made it after hacking together our own rsync-based abominations and getting really annoyed that no one had solved this genre of problems in a way that normal people could use.

Okay, I can stop shilling now. I was just excited that other people run into these problems.

I think there are many apps that do exactly what you described.

Here is one I know of:

http://www.fogcreek.com/CityDesk/index.html/